Casting is a technique of melting metal and pouring the molten metal into a die to thereby produce desired articles. Each metal has its own proper timing at which it should be poured into a die. If the molten metal is poured at a time earlier than the right time for that metal, the molten metal will not spread to corners of a cavity in the die because of its high viscosity, which prevents precise manufacturing of aimed articles. If, on the contrary, the molten metal is poured too late, its temperature becomes so high that the molten metal may be evaporated off, oxidized, or have its composition modified. Sometimes, molten metal at high temperature may stick to a die. Thus, the quality of cast articles depends greatly on the pouring time.
An exact pouring timing has correlation with the surface temperature of molten metal. It has been proposed to determine the timing for pouring molten metal into a die, based on the surface temperature of the molten metal measured by an infrared thermometer. However, the thermal infrared emissivity differs from metal to metal, and the emissivity of a particular metal changes from time to time because the surface state of molten metal changes during melting. Furthermore, when the metal melts and its viscosity begins to decrease, parts of surface films such as metal oxide films over the surface of the molten metal may float and drift on the surface of the molten metal. This causes irregular changes in infrared emissivity of the molten metal surface. Some metals may absorb infrared radiation when they evaporate or generate gas. Because of these factors, it is not easy to accurately measure the surface temperature of molten metal with stability.
The assignee of the present U.S. patent application filed Japanese Patent Application No. 2000-60845 on Mar. 6, 2000 which was laid open for public inspection under Japanese Patent Application Publication No. 2001-252758, and from which U.S. Pat. No. 6,505,675 entitled “Molten Metal Pouring Time Determining Apparatus” and issued on Jan. 14, 2003 claimed a priority date. One of the co-inventors of the U.S. patent is one of the co-inventors of the present application. What was disclosed in the Japanese application was a casting apparatus in which metal placed in a melting pot is melted by applying a high-frequency signal modulated with a low-frequency signal, and a light receiver receives light emitted by the metal in the melting pot. Since the melting of metal is carried out by means of a high-frequency signal modulated with a low-frequency signal, the molten metal vibrates in accordance with the low-frequency signal, which results in a corresponding vibrating component developed in a received-light representative signal developed by the light-receiver. The molten metal is poured into a die when the vibrating component exceeds a reference value.
In casting, unused metal is not always used solely, but unused metal is sometimes used together with metal pieces removed from a cast article with an oxide film formed thereon. In such case, the oxide film may float and move to vibrate on the surface of the molten metal, and, therefore, an accurate metal temperature cannot be determined. In other case, different-sized metal pieces are melted together. In such case, it may occur that although smaller metal pieces have melted, larger ones have not. The molten metal mass of the smaller pieces may cause the vibrating component in the received-light representative signal to exceed the reference value. Also, at the time when the molten metal mass resulting from continuous heating of the larger pieces after the smaller ones have been melted is poured into a die, part of the molten metal may be evaporated or oxidized, or the molten metal composition may change. In addition, the molten metal poured into the die may stick to the die.
An object of the present invention, therefore, is to provide a casting apparatus with which molten metal can be poured into a die at a time determined on the basis of a true temperature of the molten metal, even when different-sized pieces of metal are used or a metal piece with an oxide film formed on it is used as a starting material.